As wireless communications rapidly spread and promise to deliver voice and data to every walk of modern life, a great deal of effort is devoted to enhance the capacity and performance of wireless communication networks.
US IS-95 CDMA cellular standard, the second US digital cellular standard based on CDMA (code division multiple access) technology, is designed to deliver a very high capacity by way of spread-spectrum signal techniques. In IS-95, the downlink (or forward link) channel structure consists of the transmission of up to 64 simultaneous, distinct channels with varying functions that are orthogonally multiplexed onto a common RF carrier. One of these channels is a high-power pilot signal that is transmitted continuously as a coherent phase reference of reception of a radio frequency (RF) carrier modulated by information. Another of these channels is a continuously transmitted synchronization channel that is used to convey system information to all users in the cell. Up to seven paging channels are used to signal incoming calls to mobiles in the cell, and to convey channel assignments and other signaling messages to individual mobiles. The remainder of the channels is designated as traffic channels, each transmitting voice and data to an individual mobile user.
At the mobile station, the common broadcast pilot signal is used to recover the phase of the RF carrier in order to coherently demodulate the received traffic signals. This approach would require that both the pilot and traffic signals received at the mobile station have the same phase as the RF carrier. This phase-matching requirement is easy to accomplish if the pilot signal and the traffic signals are transmitted through the same sector beam, often through a single antenna, as illustrated in FIG. 1A. In FIG. 1A, an antenna 101 at a base station 100 transmits a sector beam carrying, among other things, a pilot signal 103 and a traffic signal designated 104 to a mobile station 102.
U.S. Pat. No. 6,108,565 describes a system and method for enhancing CDMA communication capacity. In particular, it provides a base station with an adaptive antenna array system, such that a common pilot signal is transmitted through a sector wide beam and one or more traffic signals are transmitted through a narrower adaptive spatial beam, as illustrated in FIG. 1B. In FIG. 1B, an adaptive antenna array system 111 at a base station 110 transmits a sector wide beam carrying a common pilot signal 113 and a narrower adaptive spatial beam carrying a traffic signal 114 designated to a mobile station 112 along with other traffic signals.
The aforementioned adaptive spatial beams render important advantages of providing higher effective radiated power to the desired user and reducing cross-talk interference to other users. However, this novel approach requires that the downlink traffic beam be phase matched with the sector pilot beam, in order to carry out coherent demodulation at individual IS-95 mobile stations. Moreover, care must be taken to ensure that the phase-matching process is achieved not at the expense of compromising the gain of the traffic beam and hence the gain of the downlink system capacity.
There exists a need, therefore, for a simple and effective method for matching the phases of the common pilot beam and the dedicated traffic beams without compromising the downlink system capacity.